Variometers are used in airplanes for measuring the climbing and descending speeds. In their known various designs, they comprise a measuring chamber communicating with the atmosphere and a tight measuring box located within the chamber and also communicating with the atmosphere. The measuring box is connected, through a lever system, to a pointer and is elastically deformable in a manner such that pressure differences occurring between the interior of the measuring box and the space surrounding it, i.e. the interior of the measuring chamber, effect a deflection of the pointer.
The basic idea underlying such an arrangement is to establish a communication with the atmosphere, of either the measuring box or the measuring chamber, not directly but through a retardation member. The result is that the variations of the atmospheric pressure due to the varying flight altitude cause a pressure differential between the measuring box and the measuring chamber, which is taken as a measure of the climbing or descending speed.
In the older known designs, the measuring chamber is connected to the atmosphere directly while the measuring box is connected through a retardation member. In such a design, it is not necessary to seal the measuring chamber nor the indicating mechanism mounted therein against the outside. However, to obtain a satisfactory time constant of the time delay, an equalizing receptacle is needed which is connected to the measuring box and occupies a relatively large space.
In the more recent known designs, on the contrary, the retardation member is mounted upstream of the measuring chamber while the measuring box is connected to the atmosphere directly, through a line having a very small flow resistance. In this design, of course, except for the passage formed by the retardation member, the interior of the measuring chamber must be hermetically sealed relative to the outside.
In one of the known designs, a ceramic plate having through pores is used as the retardation member. For calibrating the variometer, prior to the final assemblage, first a static calibration is performed during which definite pressure differences are produced between the measuring box interior and its exterior. Thereupon, the lever system, serving to transmit the deformation of the measuring box to the pointer, is adjusted so that, at each of the produced differential pressures, a definite, altitude-change speed corresponding thereto is indicated. Following this static calibration, the measuring chamber of the variometer is closed and the variometer is brought into a test chamber for dynamic calibration, and in which test chamber the internal pressure can be varied for simulating changes in altitude. If, now, the variometer indicates incorrect climbing and descending speeds, the flow resistance of the retardation member must be correspondingly changed to obtain the differential pressure provided for the respective rate of climb or descent. In case the flow resistance is too small, a part of the ceramic plate must be covered with varnish to close a part of the pores. On the other hand, should the flow resistance be too high, the ceramic plate must be replaced. Since the ceramic plate is not accessible from the outside of the measuring chamber, the measuring chamber must be opened for each test. It is usual that the flow resistance must be changed several times to obtain the correct indication, so that a calibration of such variometers takes a very considerable time.
In another known design, a system of orifices is used as the retardation member, comprising a plurality of orifice inserts which are disposed one after the other and each provided with a bore. Then, if the time of retardation is to be reduced during the calibration, this may be done by enlarging the bore in the first orifice insert. If, on the contrary, the time of retardation is to be increased, one of the orifice inserts must be replaced by another having a smaller bore. Therefore, in this design again, the calibration is very time-consuming.